The present invention relates to a positive temperature coefficient thermistor device and particularly to such a device adapted to withstand relatively high voltages.
Current limiting device 10 in FIG. 1 suitably comprises a positive temperature coefficient (PTC) thermistor typically taking the form of a barium titanate bulk element or slug through which electrical current passes to a sensitive measuring instrument or other protected circuit 16. The thermistor limits input current while a shunt voltage limiting device 18 is connected specifically across circuit 16 for protecting circuit 16 from excessive voltage levels. Typically, the clamping voltage of voltage limiting device 18 is much less than the rated operating voltage of current limiting device 10. Under normal operating conditions the thermistor exhibits a low resistance, but if an excessively high voltage is applied to the entire circuit including the thermistor, circuit 16 and device 18, or if a short occurs in circuit 16, the thermistor heats in response to the passage of current and its resistance increases many orders of magnitude for current limiting. A typical high voltage thermistor as commercially available is able to withstand about 660 volts appearing thereacross on a cyclic basis. However, for instruments such as digital multimeters wherein a voltage on the order of 1000 volts may be inadvertently applied across a pair of input terminals or probes, a series thermistor must be able to withstand high voltage levels.
Referring to FIG. 2, application of predetermined voltage between metalized ends of barium titanate thermistor body 10 causes the series resistance of the device to increase for circuit protection. However, a voltage higher than rated voltage may damage the thermistor. It may appear that merely lengthening the body (i.e., increasing dimension L in FIG. 2) would increase the voltage at which the device operates. Unfortunately, this is not the case. Assume a given thermistor has a length and diameter of L and D respectively, and operates reliably at a voltage Va, i.e. is capable of having a voltage Va applied and removed repeatedly. If a second thermistor is constructed with identical material and diameter, but a length 2 L, it will often not operate reliably at a voltage 2 Va.
Due to the crystalline makeup of a PTC thermistor, the manufacture of a completely homogeneous structure having the same resistivity throughout is not very likely. Therefore, instead of heating up all at once, thermistor 10 in FIG. 2 can develop a hot spot 12 particularly when a voltage above rated voltage is applied across the thermistor. The hot area exhibits increased resistance, dropping a disproportionate share of the thermistor voltage and resulting in excessive local power dissipation. The action is cumulative in positive feedback fashion. Although for normal thermistors operating within ratings, heat is quickly conducted to other parts of the body whereby a high resistance is produced which is distributed over the entire thermistor, the hot spot phenomenon effectively limits the voltage rating of the device.
Thus, if 2 Va is applied across the ends of a thermistor of length 2 L, this higher voltage may at first be primarily concentrated in the same small cross section 12 and the heat produced isn't dissipated fast enough. Failure can take place as a result of two mechanisms. First, the large thermal gradient at the hot spot can cause the thermistor to crack as a result of mechanical stress since the thermistor body is quite brittle. Secondly, the large voltage gradient at this point can cause arc over and conductive tracing on the body surface.